| Term | - | Definition |
| A Measurement | - | is the boxing system dimension, which represents the horizontal measurement of a lens or a frame's eyewire, enclosing the lens. |
| Abberation | - | the failure of a lens or mirror to bring light rays to a single focal point. |
| Abduction | - | is an outward movement of the eyes temporally. |
| Accommodation | - | is the eyes ability to focus at varying distances using the crystalline lens. |
| Addition (add power) | - | is the difference between the distance and near dioptric power of a multifocal. |
| Adduction | - | is an inward movement of the eye medially. |
| Amblyopia | - | is also known as lazy eye. A loss of vision for no apparent reason. |
| Ametropia | - | is an abnormal refractive state of the eye to include hyperopia, myopia, or astigmatism. |
| Aniseikonia | - | is a condition where each eye perceives an unequal image size compared to the other eye. This is most commonly caused by anisometropia. |
| Anisometropia | - | is a difference in refractive power of the two eyes exceeding 1.50 diopters. |
| ANSI | - | American National Standards Institute in New York. This agency provides recommended standards for many professional disciplines to include the ophthalmic profession. |
| Anti-Reflective Coating | - | (also known as thin film) is a vacuum process of depositing a thin layer(s) such as magnesium fluoride, silicon, or titanium dioxide to a lens surface to allow a higher transmission of light through the lens. |
| Antimetropia | - | is a refractive error in which one eye is myopic and the other eye is hyperopic. |
| Aphakia | - | is an eye without a crystalline lens. |
| Aqueous Humor | - | is the clear fluid in the anterior chamber of the eye which is responsible for maintaining intraocular pressure. |
| Asthenopia | - | is eye fatigue due to the over use of the ocular muscles and or accommodation. |
| Astigmatism | - | is a refractive error in which rays of light fail to come to a single focal point on the retina. Astigmatism is most commonly due to an irregular shaped cornea. |
| B Measurement | - | is the boxing system dimension which represents the longest vertical measurement of a lens or a frame's eyewire which is enclosing the frame. |
| Base Curve | - | is the single front surface of a lens measured in diopters over the distance portion of a lens. |
| Bevel | - | is the machined edge of a lens. The most common bevel is the "V" bevel. It is called a "V" bevel due to the inverted "V" shape on the lens edge. |
| Bezel | - | is the inside groove of an eyewire that secures the bevel of a lens. |
| Bi-Centric Grinding | - | (also know as slab-off) is the process of grinding base up prism to correct vertical imbalance. |
| Bi-Concave Lens | - | is a minus powered lens having both a minus base and ocular surface. |
| Bifocal | - | is a lens with two distinct focal lengths. Commonly used to correct presbyopia. |
| Bifrindgence | - | is a type of lens aberration that is induced during the manufacturing process which results in increased chromatic aberration. |
| Boxing System | - | is a method used to determine the geometric center of a frame. A letter variable is used to designate each dimension of a frame opening. See A, B, C, or ED Measurement in this glossary for more details. |
| Bridge | - | is the frame portion connecting the right and left eye wire. |
| C Measurement | - | is the boxing dimension representing the circumference of a lens. |
| Canthus | - | is the angle created by the junction between the upper and lower palpebra and eyelid. |
| Cataract | - | is an opacity of the crystalline lens that results in diminished visual acuity. |
| Center of Rotation | - | is the midway point (13mm behind the retina) in the eye in which the eye pivots or changes its visual axis. |
| Chiasm | - | is located at the lower portion of the brain and is the point at which the two optic nerves intersect. |
| Choroid | - | is a vascular intermediate membrane that nourishes the retina. |
| Chromatic Aberration | - | (also known as chromatism) is the dispersion of white light into its natural component colors after passing through a prism or lens. |
| Ciliary Muscle | - | is the muscle attached to the crystalline lens. This muscle is responsible for accommodation. |
| Cones | - | are the photoreceptor cells located in the macular region and are responsible for detecting color and acuity. |
| Conjunctiva | - | consists of two joining membranes, the palpebra conjunctiva and the bulbar conjuntiva. The first membrane lines the inner portion of each eyelid. The bulbar conjunctiva covers the unseen outer portion of the globe. |
| Conjunctivitis | - | (also known as "pink eye") is an inflammation of the conjunctiva. |
| Cornea | - | is the five layered, transparent optical membrane with approximately 43 diopters of power located on the front side of the eye. |
| Crystalline Lens | - | is a clear lens located between the iris and the vitreous. It is responsible for focusing light on the retina. It is capable of 19 diopters of focusing power. |
| Cycloplegic | - | is a ophthalmic pharmaceutical used to inhibit accomodation by relaxing the ciliary muscle. |
| Cylinder | - | (or "toric shape") a lens with curvature (power) in one direction only. Zero or plano power is present in the opposite meridian. |
| Cylinder Axis | - | is the orientation of the cylinder where zero cylinder power is present. |
| Datum-Line | - | is half the frames B measurement. |
| DBL | - | Distance Between Lenses is the closest measurement distance between the right and left lens. |
| Decentration | - | (move from the frames geometric center) is the process of moving the optical center in the frame to coincide with the patient's pupil during the layout of lenses before the edging process begins. |
| Diopter | - | is the unit of measurement used to quantify the power of a lens or mirror. One diopter is equal to a distance of one-meter. |
| Diplopia | - | is seeing one object as two (double vision). |
| ED | - | Effective Diameter is a box measurement that represents twice the longest radius of a lens or frame opening. |
| Emmetropia | - | is an eye with no refractive error present. |
| Eyewire | - | is the portion of a frame surrounding the lens. |
| Fovea | - | is a portion of the macula or depression in the retina containing the concentration of the cone photoreceptors. |
| Fresnel Prism | - | is a thin vinyl membrane that is pressed onto a lens as a temporary means of prescribing prism. |
| Geometric Center | - | is the equidistance (equal distance) point of a lens or frame opening. |
| Glaucoma | - | is an eye disease caused by a lack of outflow of the aqueous through the trabecular network meshwork. The result creates a significant increase in intraocular pressure. |
| Glaze | - | is to mount a pair of lenses into a frame. |
| Hordeolum | - | (also known as a "sty") is an infection common in children that occurs at the canthus area. |
| Hyperopia | - | is a refractive error characterized by blurry near vision. This condition is commonly known as "farsightness". If the retina were transparent, light would focus behind it. |
| Illumination | - | is the amount or intensity of light striking an object. |
| Image Jump | - | is a prismatic effect created by having two optical center in a bifocal lens. Since plus power is present in the segment portion of the lens, base down prism is induced which causes the image to be displaced upward towards the apex. |
| Intraocular Lens | - | (IOL) is an artificial lens implant post cataract. |
| Iris | - | is the fibrous circular muscle, which appears as the color of a persons eyes, with an opening in the center to form the pupil. |
| Ishihara's Test | - | is a test used to detect color deficiencies. |
| Isometropic | - | is having the identical prescription in each eye. |
| Jaeger Card | - | is a reading card with various sized fonts used at a distance of 14 to 16 inches to measure near visual acuity. |
| Keratoconus | - | is the degenerative thinning of the cornea in a cone shape causing irregular astigmatism which leads to a loss of vision. |
| Keratometer | - | is an instrument used to measure the corneas radius of curvature. |
| Lacrimal Gland | - | is located above the outer angle above the eyebrow and is responsible for producing tears. |
| Lasik | - | (Laser in-Situ Keratomileusis) is a surgical procedure using a laser to reshape the cornea to correct refractive errors. |
| Lensometer | - | is an instrument used to measure lens sphere, cylinder, prism power, and cylinder axis. Also known as a focimeter, vertometer, or lens meter. |
| Limbus | - | is the zone between the cornea and the sclera. |
| Macula | - | is positioned on the retina containing the concentration of cones (photoreceptors). |
| Magnesium Fluoride | - | is a metal oxide commonly applied to the surface of ophthalmic lenses to reduce and eliminate specular reflections. |
| Magnification | - | is the ratio between the image produced and the actual object size. |
| MBS | - | Minimum Blank Size is the smallest possible lens blank that can be edged in a frame after decentration and bevel placement is calculated. |
| Meniscus | - | (crescent shape) is a lens that has both a convex (plus) base curve and a concave (minus) ocular curve. |
| Miotic | - | are ophthalmic pharmaceuticals used to constrict the pupil. Used to treat glaucoma. (ie. carbachol, demecarium, pilocarpine 1 to 4%, and others). |
| MRP | - | Major Reference Point is the point on a lens where prism is prescribed. The MRP and the theoretical optical center coincide on lenses without prescribed prism. |
| Mydriatic | - | is an ophthalmic pharmaceutical used to dilate the pupil without affecting accomodation. |
| Myopia | - | is a refractive error characterized by blurry distance vision. This is commonly known as a nearsighted eye. Light focuses in front of the retina. |
| n | - | is the variable used to represent a materials index of refraction. |
| Nanometer | - | is a unit of measurement equaling one-billionth of one meter. |
| Nystagmus | - | is an uncontrolled rapid movement of the eye. |
| Ocular Surface | - | (or anterior surface) is the back surface of a lens. |
| Oculus Dexter | - | (O.D.) is Latin for right eye. |
| Oculus Sinister | - | (O.S.) is Latin for left eye. |
| Oculus Uterque | - | is Latin for each eye. |
| Optic Chiasm | - | is where the optic nerve from each eye intersects in the brain. |
| Optic Disc | - | is on the optic nerve head. It is the point where the optic nerve enters the eye. A scotoma (or blind spot) is present at the optic disc. |
| Optic Nerve | - | is the nerve that transmits visual impressions to the brain. |
| Optical Axis | - | is the line that passes directly through the centers of curvature of a lens or mirror. |
| Optical Cross | - | is a graphical representation of meridian of lens power. |
| Palpebra | - | is the eyelid. |
| Pantoscopic Tilt | - | is the angle formed when the lower eyewire is closer to the patient's face than the top of the frame. Approximately 10 to 12 degrees is desirable. |
| PD | - | interpupillary distance is the distance between each pupil measured in millimeters. |
| Phoria | - | is a tendency for the eyes to deviate from their normal position. |
| Phoropter | - | is an instrument containing a series of lenses and other devices for determining the eyes refractive state. |
| Photochromatic Lenses | - | are lenses that have the ability to change the amount of light transmittance (get darker) when exposed to ultraviolet light. |
| Photophobia | - | is the sensitivity to light. |
| Photoreceptors | - | (cones and rods) light receiving cells located on the macular region of the retina. |
| Plano | - | is a term used to indicate zero or no dioptric power. |
| Polarized Lens | - | is a lens with a polarizing filter which blocks light in one meridian yet transmits light in the opposite meridian. Used in suglasses to block glare. |
| Prentice Rule | - | is the formula used to quantify and calculate prism, named after its inventor Charles F. Prentice. |
| Presbyopia | - | is a condition characterized by diminished accommodation due to the loss of elasticity of the ciliary muscle. |
| Prism | - | is any transparent material in the shape of a wedge having a thin edge (apex) and a thick portion (base). |
| Progressive Lens | - | is a multifocal lens that gradually increases its plus power from the lens' upper portion to the bottom portion. |
| Psuedophakia | - | is an eye with an artificial lens (crystalline lens) implant. |
| Pterygium | - | is a triangular membrane that covers a portion of the cornea and is a result of excessive exposure to wind and debris. |
| Ptosis | - | refers to a drooping of the upper eyelid. |
| Ptosis Crutch | - | A small wire attached medially (or nasally) to a frame designed to hold up a drooping eyelid. This device is beneficial for patients diagnosed with Ptosis Palpebra. |
| Pupil | - | is the opening through the iris, which regulates the amount of light entering the eye. |
| Rectus Muscles | - | are the six extrinsic muscles attached to each eye responsible for eye motility. |
| Regular Astigmatism | - | A refractive error in which rays of light come to two distinct line focal points and is correctable using cylindrical lenses. |
| Retina | - | is a light sensory receptor located on the inside back portion of the eye. |
| Retinitis Pigmentosa | - | (RP) is a hereditary disease affecting the rod photoreceptors on the retina characterized by restricted visual fields and night blindness. |
| Retroscopic Tilt | - | is the opposite of pantoscopic tilt. It is the angle created, when viewed from the side, between the vertical and the frame front. The bottom portion of the eyewire is adjusted further from the face compared to the upper eyewire. |
| Sagittal Depth | - | is the difference between the lens edge at the thickest point and the center of the ocular curve. (abbreviated "sag"; Latin for "arrow") |
| Sclera | - | is the oter fibrous protective structure which is incessant with the cornea. |
| Scotoma | - | a blind spot. |
| Segment | - | is the portion of a multifocal containing the additional plus power. |
| Slit Lamp | - | is an instrument that uses beams of light to inspect the front portion of the globe. Typically used with a 20D, 28D, 40D, or 90D condensing lens. |
| Snellen Chart | - | a chart used to test and measure distance visual acuity. |
| Stereopsis | - | is depth perception that is dependent on binocular vision. |
| Strabismus | - | is a condition when the eyes are not straight. This condition is most commonly caused by an extraocular muscle imbalance. |
| Stye | - | (hordeolum) an inflammation on the margins of the eyelids (common in children). |
| Suspensory Ligaments | - | Ligaments responsible for the suspensory apparatus of the crystaline lens. Made up of a number of thin tissue strands that connect the ciliary processes to the lens equator. |
| The Angle | - | refers to the area where the cornea and iris intersect. At this point is the filtering tissue known as the "trabecular meshwork". |
| Tonometer | - | is an instrument used to measure intraocular pressure. |
| Trabecular Meshwork | - | refers to the filtering tissue located in the area where the iris and cornea intersect. |
| Trifocal | - | is type of multifocal most commonly used to provide distance, intermediate, and reading correction. |
| Tropia | - | is a distinct deviation of the eye(s) from the visual axis due to a lack of fusion. There are four basic tropias designated by the direction of deviation: esotropia (eye turns inward), exotropia (eye turns outward), hypotropia (eye turns downward), and |
| Vertex Distance | - | is the distance measured in millimeters from the cornea to the ocular surface of a lens while the glasses are being worn. |
| Visual Acuity | - | is the relative measurement of a person's clarity of vision. |
| Visual Axis | - | is the eyes angle of gaze while looking through spectacle lenses which may or may not be in line with the optical axis of the lens. |
